Trying to understand your thing about the Cake twins being older made my brain hurt.

It's just... it made no sense to me, the way they appear in the show they are too big for a newborn, and they already had grown their mane and tail.

It's because there really isn't a way to draw a newborn horse (or anything for that matter) accurately and still have it look cute. If they drew them the right way, they'd be all wrinkly and gross, so they took some creative liberties.

Besides that, in the scene at the beginning of the episode, which takes place immediately after the two are born, they pretty clearly have hair.

I feel like it would ultimately make more sense to assume that the period of adolescence shorter for the ponies than for people (which would also help to explain the fact that in Equestria Girls, the characters are in high school but in the main series, they seem to be grown adults).

Meh... unfortunately for you, I prefer realism rather than cuteness. It's less painful to imagine small uncute wrinkly furless newborn ponies that imagining they really were born like that, in a size and shape that would destroy their mother uterus.

Sorry... really sorry...